Cooling requirements for internal combustion engines are subject to wide variations depending upon operating conditions. When the engine is cold or cool, little or no cooling is required. Indeed, in very cold operating conditions, cooling can be undesirable. During engine operation, the degree of cooling required varies as a function of engine load, and with external conditions such as air temperature and wind or vehicle velocity.
The conventional internal combustion engine vehicle is provided with an engine driven fan. The fan can absorb a considerable proportion of the total output power of an engine. For example, in large diesel over-the-road tractors, the fan might require as much as 55 horsepower. With that in mind, and considering the requirements for fuel efficiency, fan drives have been developed which operate only when cooling is needed. Thus, when the engine is operating in normal load and at cruising speeds, there can be adequate air flow through the radiator without the fan, to allow the fan to be declutched. In stop-and-go traffic, under heavy load, or when parked and idling, the clutch can be engaged to couple the fan to the engine and provide cooling air flow when the coolant temperature demands.
If space were not a problem, it would be relatively straightforward to provide a continuously variable relatively reliable clutch mechanism to associate with the fan drive in a diesel tractor. However, when one works "under the hood" and appreciates the desires of truck and engine designers to efficiently use that under-the-hood space, one soon appreciates that a relatively small envelope is available for the clutch mechanism. The envelope is limited axially by the distance between the radiator and the engine, and it is limited radially, as a practical matter, by the size of the sheave which can be accommodated for the pulley drive which conventionally drives the fan.
The clutch mechanisms which have been used heretofore, have not been without their problems. One approach is to utilize a dry clutch, but that typically results in on/off operation, since the dry clutch could not slip for long without overheating. Inherent in on/off applications is the typical shock load to the drive unit when the drive clutch is engaged. The shock load is not only undesirable from the viewpoint of loading and wear on the drive components, but is also aesthetically detrimental. For example, when the vehicle is parked during a driver's rest, but the engine is running in order to maintain heat or cooling, the fan clutch will typically cycle on and off. Even with good sound insulation between the engine compartment and the cab, the fan cycling can create a significant periodic audible disturbance.
One attempt to avoid these problems with dry clutch fan drives has been the attempted use of viscous coupling between the input and output members of the drive unit. These approaches have also had their drawbacks. First of all, viscous couplings have poor release capability and no lock-up capability. In other words, a viscous coupling will not permit the drive input and output members to be driven at the same speed. Moreover, fan drives using viscous couplings have limited horsepower capability, and cannot quickly dissipate heat buildup in the unit. Most viscous coupling designs are slow to engage after sensing heat, and cannot be locked in the off position when cooling is not desired.
Certain approaches have been developed for using wet clutches in fan drives. However, wet clutches with adequate horsepower for fan drive operation have been less compact than desired for some applications. For example, in off-the-road vehicles, such as tractors, loaders, graders and the like, there is adequate room in the engine compartment to tolerate a clutch drive mechanism of the size associated with a typical wet clutch device.
The wet clutch mechanism is very desirable in that it can provide relatively continuously variable speed, and will not, by virtue of the oil bathed clutch mechanism, overheat under most conditions. The wet clutch mechanism is typically operated from oil in the engine sump, and particularly when the vehicle is of the type which has a relatively wide range of engine operating rpm's, and therefore a relatively wide range of operating oil pressures, the clutch mechanism must be designed with sufficient operating area for the hydraulic piston to effectively operate the clutch at the oil pressure extremes. That usually requires a rather large piston area associated with the clutch so that reliable operation can be achieved from relatively low oil pressures at idle to relatively high oil pressures at high engine rpm. While those problems have proven to be solvable in off-the-road type vehicles where space is important but not at a significant premium, there are applications where the space requirements can pose a problem.
For example, in over-the-road diesel tractors, the requirements for aerodynamics, appearance, vehicle size, vehicle weight, and the like have all combined to reduce the size of the engine compartment and pack that compartment with operating components. As a result, the envelope available for installation of a fan clutch mechanism in the over-the-road vehicle is not large. That envelope has typically accommodated a dry clutch on/off clutch mechanism, and control wiring has been such as to allow a single wire to an air control valve which controls the fan clutch. Thus, in the typical over-the-road vehicle, a small and relatively reliable dry clutch mechanism is used in an on/off mode, with the problems inherent in such a limited device.
It would be desirable to provide continuously variable speed control of the clutch mechanism, but without a redesign of the engine compartment needed to accommodate a larger clutch mechanism. The provision of variable speed control would indeed provide enhanced engine operating efficiency in driving the fan only so fast as is needed for a particular set of cooling requirements, and would do away with the shock loads and audible disturbances inherent in on/off operation. However, heretofore it has not been possible to achieve the combination of small envelope, reliable operation and continuously variable drive in a long life clutch mechanism.